The invention is based on a priority application EP 01 440 264.8 which is hereby incorporated by reference.
The present invention relates to the field of optical fiber telecommunications. More specifically, the subject of the invention is a slanted Bragg grating optical fiber and a process for manufacturing such a fiber.
It is known to try to flatten the gain of optical amplifiers, such as erbium-doped fiber amplifiers, generally used along multichannel long-haul optical links. This is because the profile of the gain as a function of wavelength of such amplifiers is not constant, certain components being preferentially amplified.
Bragg grating fibers are known in the field of filtering optical fibers. A Bragg grating is formed by approximately periodic modulations of the refractive index in the core and/or in one or more claddings of the generally monomode fiber, said modulations being obtained by irradiation of the fiber by means of UV beams, also called UV exposure. The period of the grating, that is to say the period of modulation of the index along the fiber, i.e. along its longitudinal axis, and the modulation amplitude fix the spectral response of the grating.
When the index variations are inclined at a writing angle α with respect to a normal plane to the fiber axis an SBG (Slanted Bragg Grating) is formed.
Patent application EP-0855608 discloses one method of producing an SBG used in transmission and intended to compensate for a lack of flatness around 1558 nm of a broadband EDFA optical amplifier. The SBG has the advantage of being non reflecting at filtering wavelengths as a long period Bragg grating (LPG) and is less sensitive to temperature variations and fiber bending than a LPG.
The inclination of the period at a nonzero writing angle α has the effect of coupling the copropagating fundamental mode which propagates in the core of the fiber to the dissipative cladding contrapropagating modes. The transmission spectrum of the SBG therefore corresponds to a spectral envelope of all of the components associated with these various cladding modes. This results in dissipation of the guided optical power in the fiber, manifested by a selective attenuation of the transmitted signals. It is possible in this way to correct the overamplifications at certain wavelengths of the optical amplifier.
Depending on the length of the fiber section exposed to the UV radiation, on the value of the period along this section, on the intensity and the angle of the exposure and the fiber profile, the following transmission characteristics may be modified: the bandwidth, the central wavelength, which is slightly less than the Bragg wavelength, and the amplitude of the selective attenuation. These parameters are set depending on the desired equalization and the transmission window of the telecommunication system involved.
As an example, the document entitled “36-nm Amplifier Gain Equalizer Based On Slanted Bragg Grating Technology For Multichannel Transmission” published in SubOptic 2001, Japan, P 4.3.10 discloses three SBGs cascaded randomly disposed around the fiber axis and one after another. Each grating has different characteristics (writing angle, period) from the other gratings so as to adjust the desired gain equalization.
However, these SBGs of the prior art have the drawback of being sensitive to the state of polarization of the fundamental propagation mode LP01. More specifically, they involve losses linked to the polarization state denoted by the name PDL (Polarization-Dependant Loss). It is possible to define the PDL as the difference in attenuation between an s-type polarization and a normal p-type polarization in the transverse plane of the electric field E of the LP01 mode. The PDL losses are different depending on the wavelength: the PDL spectrum as a function of wavelength has a peak and a width substantially equal to the width of the SBG transmission spectrum. Thus, the SBG transmission spectrum is linked to the state of polarization of the LP01 mode.
It is known that the larger is the PDL the larger is the angle of writing α of the SBG. Likewise, the higher is the level of attenuation introduced by the SBG so-called contrast, the higher is the PDL. At the present time, the PDL of an SBG inclined at approximately 7.2° is, for example, 0.26 dB around 1552 nm for a contrast of −3.5 dB. In this case, the PDL spectrum has a width at mid-height of the order of 16 nm.
Endeavors are being made to reduce the PDL especially for the next generations of gain equalizers intended to be used on very long-haul submarine links.